1. Field of the Invention
This invention relates to a process for preventing foaming in cationic electrophoretic coating using special polyalkylene oxide compounds which have a solubility of more than 50 parts by weight in 100 parts by weight of water. The invention also relates to electrophoretic coating baths containing special polyalkylene oxide compounds which are distinguished by a particularly low tendency to foam on agitation.
2. Discussion of Related Art
The coating of conductive substrates by cataphoresis or even cathodic electrodeposition coating has been adopted in almost all areas of mass painting. In most countries, its main application is in the automotive industry where the major advantages of this coating process are particularly relevant. Thus, substantially defect-free uniform coatings can be obtained by dip coating and even the coating of voids is readily possible.
In cathodic electrodeposition coating (CEDC), the substrate is dipped into an aqueous bath, the so-called electrodeposition bath or ED bath, and connected as the cathode. By applying a direct current, the binder is then deposited from the bath onto the substrate. The binder thus deposited is then hardened by baking or other methods.
Cationic electrophoretic coating (CEC) installations are extended dip coating installations. The workpiece is provided with electrodes and coated in the dip tank over a contact time of about 1 to 4 minutes at voltages of 100 to 500 V and current densities of 0.01 to 5 A/dm.sup.2. In the automotive industry, for example, the tank volumes are between 50 and 450 ton.
After returning to the surface, the coated objects pass through a rinse zone to remove any non-deposited lacquer film and then through blowing and drying zones.
The production and use of aqueous polymer systems is often accompanied by foaming. The foam is produced through the presence of surfactants and stabilized. Corresponding surface-active substances are present in virtually every water-based paint or lacquer formulation. Thus, this group of substances includes, for example, emulsifiers, wetting agents, dispersants and polymeric surfactants, such as for example the charged binders used in CEC coating compositions. Surfactants develop their effect by migrating to the phase interface between two generally immiscible or substantially immiscible components where they reduce surface tension. However, a side effect of this mechanism--necessary for the production of dispersions--is its tendency to stabilize and disperse trapped gas bubbles, especially air bubbles. Accordingly, the introduction of gases into a surfactant-containing solution often results in foaming. Foaming can take place in various ways, for example during production when the coating composition is thoroughly mixed or during the application of the coating composition. Thus, in the case of electrophoretic coating, air can be introduced in dispersed form, for example during the introduction or removal of the workpiece, during the washing of the workpiece and during filling of the dip tank with coating composition, often leading to the formation of a voluminous, stable foam in coating compositions known from the prior art.
Depending on the stability of the foam, foaming such as this can lead to unwanted effects, for example overflowing of the dip tank, surface soiling of the workpieces, uneven application of the paint and, hence, breaks in production.
This problem has been overcome by adding defoamers to coating compositions. A defoamer normally contains substances which intervene in the stabilizing mechanisms of the foam. The active substances normally used develop their effect by spreading out at the original phase interface as a result of their incompatibility. Another mechanism for destroying unwanted foam consists in the absorption of surfactants at the phase interface by introduction of hydrophobic silica. In many cases, a combination of these mechanisms is also achieved in mixed preparations. For example, aliphatic and aromatic mineral oils are used as carrier liquids for transferring active substances to the otherwise hydrophilic medium. The choice of these active substances is determined by the desired range of activity. Whereas a mineral-oil-based defoamer is suitable for low-gloss to medium-gloss acrylic or styrene/acrylic lacquers or for water-based emulsion polymers, defoamers of this type can cause a distinct reduction in gloss in high-gloss lacquers.
According to Journal of Coatings Technology, Vol. 66, 47 et seq. (February 1994), a defoamer must always have a certain degree of incompatibility with the medium to be defoamed because otherwise it will not migrate to the phase interface to destroy the foam micelle. However, the disadvantage of this incompatibility lies in the danger that surface defects, such as fish eyes and craters, can be formed by these very additives. Another problem caused by incompatibility lies in the complicated incorporation of the defoamer in the medium to be defoamed. In cases of high incompatibility, the defoamer has to be mechanically incorporated in the medium, the shear rate applied during preparation of the dispersion being a very important factor. At excessive shear rates, the defoamer droplets can become too small and lose their effect so that the defoamer has to be used in larger quantities. If the shear rate is too low, so that large defoamer droplets are produced, the resulting coating composition tends to form craters and to flow unevenly.
Accordingly, there is a need for a defoamer system which has a good defoaming effect, which does not adversely affect film formation on the surface to be coated and which can be incorporated without difficulty.
EP-B-339 795 describes a method for increasing the film thickness obtainable in electrophoretic coating. End-capped polyethers with a solubility in water of 0.1 to 50 parts by weight in 100 parts by weight of water are added as film-forming additives. According to the teaching of EP-B-339 795, film formation is disrupted by higher solubility values. This document has nothing to say about the incorporation behavior and foaming behavior of the coating compositions.
JP-B-06/045916 relates to a process for washing workpieces which have been painted by electrophoretic coating. The washing liquid used is the regenerated liquid obtainable after ultrafiltration of the bath contents which contains polypropylene glycol with a molecular weight of 500 to 1,500 in a quantity of 5 to 10,000 ppm as defoamer. The document in question does not mention the use of ethylene-oxide-containing copolymers in the coating composition itself.
JP-A-01/069678 relates to a process for improving the levelling of coatings obtainable by electrophoretic deposition. A polyalkylene glycol with a molecular weight of 1,500 to 6,000 is added to the coating composition in quantities of 0.25 to 7% by weight. The solubility of the polyalkylene glycol in water, the incorporation behavior of the flow controller and the foaming properties of the resulting solution are not mentioned.
WO 90/07967 discloses antifoam formulations containing 33 to 89% by weight of a C.sub.16-22 fatty acid (mono-to-penta)ethoxylate or (mono-to-penta)propoxylate and 67 to 11% by weight of a block polymer of propylene oxide with ethylene oxide having a molecular weight of about 3800, a cloud point (expressed as 10% aqueous solution) of 9.degree. C. to 13.degree. C. and an HLB value of about 1 and their use in the paint-processing industry, more particularly in the painting of motor vehicles, for inhibiting foam in the circuit water of wet separators for spray painting installations.
U.S. Pat. No. 5,124,074 describes antifoam formulations containing a compound obtainable by reaction of special monosubstituted polyalkylene glycols containing oxyalkylene units with a polyepoxide compound containing a polyalkylene oxide unit.
The book entitled "Elektrophorese Lacke" (Kurt Weigel; Stuttgart 1967, pages 197-199) states: "To avoid unpleasant side effects, antifoam agents also have to be chosen with great care." (page 198, lines 2 to 4). In addition, it is emphasized that, in general, silicone defoamers are unsuitable in electrophoretic coating and that it is always important to ascertain which defoamer develops optimal activity for a certain system (page 198, first paragraph). These statements confirm the well-known fact that, because a certain class of compounds are suitable as defoamers in electrophoretic coating, this does not necessarily mean that other classes of compounds are fit for the same purpose. In particular, it is clear to the expert from the cited work that a given compound with a certain basic defoaming effect is not automatically suitable as an antifoam agent in cathodic electrophoretic coating.